1. Field of the Invention
The present invention relates to synthesis of 18F-labeled probes for positron emission tomography (“PET”). More specifically, a method of synthesizing 18F-labeled probes is disclosed, where a solvent with a predetermined amount of water in at least one organic solvent is used to a) elute the 18F-fluoride from an anion exchange cartridge and b) perform the 18F-labeling, without drying the 18F-fluoride, in the presence of at least one labeling reagent and at least one phase transfer catalyst.
2. Description of Related Art
Synthesis of 18F-labeled probes for positron emission tomography (“PET”) has increased tremendously over the last 10 years as there is a growing demand for radiopharmaceuticals that successfully detect aberrant biochemical functions in vivo. The unique physiochemical properties of PET tracers make them ideally suited for several imaging applications such as the early detection and staging of diseases, treatment monitoring and stratification of patients who may or may not respond to a particular therapy.
The synthesis of these radiolabeled molecules is undoubtedly time consuming, labor intensive and randomly unreliable. In an effort to minimize these production issues, radiochemists have attempted to reduce the labeling procedures to their simplest, quickest and most reliable protocols. Despite these process improvements, the radiolabeling processes still contain inherent inefficiencies that would benefit from further chemistry and process improvements.
The conventional means for 18F-labeling involves the formation of “activated” or “naked” fluoride, i.e. fluoride that is sufficiently moisture-free and thus suitable for radiolabeling. It is widely known that the desolvation of fluoride increases its nucleophilic character. See V. M. Vlasov, “Fluoride ion as a nucleophile and a leaving group in aromatic nucleophilic substitution reactions”, J. of Fluorine Chem., vol. 61, pp. 193-216 (1993). In these conventional labeling protocols, trace amounts of 18F-fluoride are sequestered onto an anion exchange column from several milliliters of 18O-water. Afterwards, the 18F-fluoride ion is eluted from the anion exchange column through the use of salts, such as K2CO3, dissolved in water. An additive such as the potassium crown ether Kryptofix K222, which is dissolved in anhydrous acetonitrile, may be used in conjunction with aqueous K2CO3 to facilitate the elution of 18F-fluoride, or optionally added into the reaction vessel after the K2CO3-mediate elution. After the elution step, there is an extensive drying protocol needed as reagents K2CO3 and Kryptofix K222 are in a highly hydrous solution of acetonitrile. This drying step generates an activated mixture of K2CO3, Kryptofix K222 and 18F-fluoride. The drying process begins by evaporating the azeotropic mixture at elevated temperatures, oftentimes at reduced pressures to aid in the evaporation of water from the reaction vessel. This initial drying can take up to 30 minutes to complete, depending on the efficiency of drying. After the first evaporation, it may be necessary to perform additional evaporations to effectively remove of enough water to render the 18F-fluoride sufficiently moisture-free for labeling.
There are several inherent problems with this approach to generating activate reagents for 18F-fluorination. First, the amount of water present after the initial drying step will vary from run to run given mechanical differences in vacuum, gas flows, valve integrity and temperature control. Any single mechanical problem, or combination thereof, will negatively impact the efficiency of drying and hence, the labeling results. Since the amount of residual water could vary greatly from run to run, the radiolabeling results would then be inconsistent, making reliable production of radiotracers difficult. Also, given the time needed to successfully dry the fluoride, a good portion of the total synthesis time is dedicated to the drying step. Lastly, because of the concern of residual water in the reaction, there is a potential for operators to “overdry” the reaction mixture prior to fluorination. In this instance, drying the reagents for too long may be as equally hurtful as under-drying the reagents (under-drying being the failure to remove sufficient moisture from the reagents for 18F-fluorination). For example, Kryptofix K222 decomposition is directly related to drying times and temperatures: prolonged drying at high temperature compromises the integrity and functionality of Kryptofix K222. To address these issues, a method that minimizes the length of drying and can accurately control the amount of moisture from run to run would be a substantial improvement to current radiolabeling practices.
Alternate methods have been developed in an attempt to obviate the need for the drying step that either elute 18F-fluoride from anion exchange resins using additives in either anhydrous organic solvents (such as acetonitrile, see Joël Aerts et al., “Fast production of highly concentrated reactive [18F] fluoride for aliphatic and aromatic nucleophilic radiolabeling”, Tetrahedron Letters, vol. 51, pp. 64-66 (2009); International Patent Application Pub. No. WO 2009/003251) or by using ionic liquids in hydrous acetonitrile (Hyung Woo Kim et al., “Rapid synthesis of [18F]FDG without an evaporation step using an ionic liquid”, Applied Radiation and Isotopes, vol. 61, pp. 1241-1246 (2004)). For these types of elutions using compounds with unknown toxicities, one would want to assay for these additives in the final product prior to injection and imaging, which ultimately complicates the production workflow.
The use of hydroalcoholic (i.e. protic solvents) co-mixtures is reported to improve 18F-labeling yields over the standard single solvent 18F-labeling conditions. Dong Wook Kim et al., “A New Class of SN2 Reactions Catalyzed by Protic Solvents: Facile Fluorination for Isotopic Labeling of Diagnostic Molecules”, J. Am. Chem. Soc., vol. 128, no. 50, pp. 16394-16397 (Nov. 23, 2006). While the increases in yields are believed to be a result of the unique interactions between the 18F-fluoride and possibly the leaving group on the precursor, it is not practical to use hydroalcoholic solvents, such as t-amyl alcohol, as they must be analyzed in the final product. Additionally, the low polarity of these bulky solvents can hinder the precursor's solubility which can be used for the labeling reaction, thus negatively impacting the radiolabeling yield.